1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring the amount of paint coating used to form one or a plurality of coated layers on a substrate such as a galvanized steel.
2. Description of the Prior Art
In general, the composition of elements forming a coating on a substrate such as, for example, at least one painted layer, is kept confidential as a know-how technology. Accordingly, an attempt has hitherto been made to measure the amount of the coating, that is, the painted layer, by irradiating a sample having a painted layer formed on a substrate and then measuring the intensity of Compton scattering rays produced from the sample, that is, radiation scattered from the sample under the well-known Compton effect. Examples of this attempt are disclosed in the Japanese Patent Publication No. Sho-63-19004 and the Japanese Laid-open Patent Publication No. Sho-64-41810.
However, since the Compton scattering rays may generate not only from the painted layers, but also from the substrate, the intensity of the Compton scattering rays as measured tends to decrease, as shown in FIG. 12(a), if even though the amount of paint coating forming the painted layer remains the same the amount of plated zinc forming a part of the substrate is large. Also, the intensity of the Compton scattering rays may differ depending on the type of material used for the plating as shown in FIG. 12(b) even though the amount of the plating material remains the same. Because of the reason mentioned above, the prior art measuring method such as disclosed in any one of the Japanese patents referred to above has managed to measure the amount of paint coating forming the painted layer with respect to the background component generating from the substrate.
However, even though the material for (or the type of) the substrate remains the same, the composition thereof, that is, the content of elements forming the substrate, may vary depending on the lot of products. In particular, in the case of the substrate comprising a galvanized steel, it is well recognized as being extremely difficult to avoid a varying thickness of a galvanized layer from one steel to another or from one lot of galvanized steel to another, resulting in an uneven amount of zinc coating.
Accordingly, the measurement of the amount of the paint coating with the use of a calibration curve appropriate to the material (type) of the substrate and/or the predetermined amount of coating of plating material (which amount is different from the actual amount of coating of plating material) often results in a measurement error due to a variation in content of the elements referred to above and/or amount of coating of the plating material.
In order to eliminate the problems discussed above, a measurement method can be contemplated which comprises irradiating a selected region of a green substrate (i.e., a substrate before a paint is applied thereto), determining a background component beforehand in reference to fluorescent X-rays produced from the green substrate, irradiating again the predetermined region of the painted substrate, that is, the substrate which has been painted, determining the intensity of Compton scattering rays produced from the painted substrate and calculating the amount of paint coating forming the paint layer with the use of the measured intensity of the Compton scattering rays and the measured background component.
According to this contemplated method, the irradiation to the predetermined region of the substrate has to be carried out twice, that is, before and after the paint application. Therefore, not only is this indeed complicated and time-consuming, but also an exact alignment of the predetermined region of the substrate before and after the paint application with the direction of irradiation is extremely difficult to achieve, and therefore, a measurement error is inevitable to a certain extent.
Specifically, where the sample to be analyzed is moving on a production line, the measurement system is required to track the sample to be analyzed. In such case, a change in speed of movement of the sample to be analyzed is apt to result in an error in measurement.
In view of the foregoing, the last-mentioned Japanese publication, that is, the Japanese Laid-open Patent Publication No. Sho-64-41810, discloses a method which comprises irradiating primary X-rays to a surface of a painted layer on a sample to be analyzed, simultaneously measuring the intensity of the Compton scattering rays and the fluorescent X-rays produced from a substrate (a plated layer) and calculating the amount of paint coating forming the painted layer on the substrate in dependence on the respective measured intensities of the radiation. However, while the fluorescent X-rays scattering from the substrate are largely absorbed by the painted layer, the above mentioned prior art has; failed to take into consideration the amount of the fluorescent X-rays absorbed by the painted layer. Because of this, information concerning the substrate, that is, the accuracy of measurement of the amount of plating material on the substrate, is extremely inaccurate and, consequently, no improvement can be found in measurement accuracy as compared with the measurement of the amount of paint coating with the use of the calibration curve appropriate to the predetermined amount of coating of plating material.
Apart from the foregoing discussion of the prior art, in the case where a painting material is applied to a plated steel, it is a general practice to apply the painting material over an outer surface of a primer. In such a case, given an amount of coating of the primer, it is possible to determine the amount of coating of the painting material over the primer by subtracting the amount of coating of the primer from the total amount of coatings including the primer and the painting material. However, the amount of coating of the primer inevitably varies from one galvanized steel to another and, on the other hand, the intensity of the Compton scattering rays scattering from the primer is, as shown in FIG. 12(c), higher than the intensity of the Compton scattering rays, scattering from the painted layer and, therefore, an error in measurement of the amount of coating of the primer coated is cumulative to an error in calculating the amount of coating of the painting material used to form the painted layer.